Multi-state magnetoresistance random access cell with improved memory storage density

ABSTRACT

A multi-state magnetoresistive random access memory device comprising a pinned ferromagnetic region having a magnetic moment vector fixed in a preferred direction in the absence of an applied magnetic field, an non-ferromagnetic spacer layer positioned on the pinned ferromagnetic region, and a free ferromagnetic region with an anisotropy designed to provide a free magnetic moment vector within the free ferromagnetic region with N stable positions, wherein N is a whole number greater than two, positioned on the non-ferromagnetic spacer layer. The number N of stable positions can be induced by a shape anisotropy of the free ferromagnetic region wherein each N stable position has a unique resistance value.

FIELD OF THE INVENTION

[0001] This invention relates to semiconductor memory devices and, moreparticularly, the present invention relates to semiconductor randomaccess memory devices that utilize a magnetic field.

BACKGROUND OF THE INVENTION

[0002] Traditional semiconductor memory devices store a memory state bystoring an electronic charge. However, magnetoresistive random accessmemory (hereinafter referred to as “MRAM”) devices store a memory stateby utilizing the direction of the magnetic moment vector created in amagnetic material or structure. Thus, a memory state in a MRAM device isnot maintained by power, but rather by the direction of the magneticmoment vector. To be commercially viable, however, MRAM must havecomparable memory density to current memory technologies, be scalablefor future generations, operate at low voltages, have low powerconsumption, and have competitive read/write speeds.

[0003] In previous MRAM technology, storing data is accomplished byapplying magnetic fields and causing a magnetic material in a MRAMdevice to be magnetized into either of two possible memory states. Thus,a single MRAM device typically stores one bit of information and toincrease the memory density, the MRAM device must be scaled laterally tosmaller dimensions.

[0004] As the bit dimension shrinks, however, three problems occur.First, the switching field increases for a given shape and filmthickness, requiring more current to switch. Second, the total switchingvolume is reduced so that the energy barrier for reversal, which isproportional to volume and switching field, drops. The energy barrierrefers to the amount of energy needed to switch the magnetic momentvector from one state to the other. The energy barrier determines thedata retention and error rate of the MRAM device and unintendedreversals can occur due to thermal fluctuations if the barrier is toosmall. Finally, because the switching field is produced by shape, theswitching field becomes more sensitive to shape variations as the bitshrinks in size. With photolithography scaling becoming more difficultat smaller dimensions, MRAM devices will have difficulty maintainingtight switching distributions.

[0005] Accordingly, it is an object of the present invention to providea new and improved magnetoresistive random access memory device whichcan store multiple states.

SUMMARY OF THE INVENTION

[0006] To achieve the objects and advantages specified above and others,a multi-state MRAM cell is disclosed. In the preferred embodiment, themulti-state MRAM cell has a resistance and includes a multi-state MRAMdevice sandwiched therebetween a first conductive line and a baseelectrode. Further, a second conductive line is positioned proximate tothe base electrode. In the preferred embodiment, the multi-state MRAMdevice includes a pinned synthetic anti-ferromagnetic region positionedadjacent to the base electrode. The pinned synthetic anti-ferromagneticregion includes an anti-ferromagnetic pinning layer and a pinnedferromagnetic layer which has a pinned magnetic moment vector orientedin a preferred direction at a first nonzero angle relative to the firstconductive line. Further, a non-ferromagnetic spacer layer is positionedon the pinned synthetic anti-ferromagnetic region.

[0007] A free ferromagnetic region is positioned on thenon-ferromagnetic spacer layer and adjacent to the second conductiveline. The free ferromagnetic region has a free magnetic moment vectorthat is free to rotate in the presence of an applied magnetic field and,in the preferred embodiment, has a shape designed to allow more thantwo, e.g. four, stable states, as will be discussed presently.

[0008] In the preferred embodiment, the free magnetoresistive regionincludes a tri-layer structure that includes an anti-ferromagneticcoupling spacer layer sandwiched therebetween two ferromagnetic layers.Further, the purpose of the first conductive line is to act as a bitline and the purpose of the second conductive line is to act as a switchline. These conductive lines supply current pulses to the MRAM device toinduce a magnetic field for aligning the free magnetic moment vector ina desired state.

[0009] The multiple states of the MRAM device are created by an inducedanisotropy within the free ferromagnetic region. In the preferredembodiment, the shape of the free ferromagnetic region is chosen tocreate multiple magnetic states wherein a first hard axis is orientedparallel with the first conductive region and a second hard axis isoriented parallel with the second conductive region. Consequently, thefree magnetic moment vector will not be stable in either of these twodirections.

[0010] Also, in the preferred embodiment, the shape of the freeferromagnetic region is chosen so that a first easy axis and a secondeasy axis are both oriented at nonzero angles to the first hard axis,the second hard axis, and the pinned magnetic moment vector. The firsteasy axis and the second easy axis are also chosen to be oriented at a90° angle relative to each other. The first easy axis creates a firststable position and a third stable position wherein the first stableposition and the third stable position are oriented anti-parallel alongthe first easy axis. The second easy axis creates a second stableposition and a fourth stable position wherein the second stable positionand the fourth stable position are oriented anti-parallel along thesecond easy axis.

[0011] Thus, in the preferred embodiment, four stable positions havebeen created by the shape induced anisotropy of the free ferromagneticregion. However, it will be understood that other methods can be used tocreate more than two stable positions in the free ferromagnetic region.Further, the resistance of the MRAM device depends on which stableposition the free magnetic moment vector is aligned with because eachstable position is oriented at a unique angle relative to the pinnedmagnetic moment vector. Hence, the four stable positions can be measuredby measuring the resistance of the MRAM device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The foregoing and further and more specific objects andadvantages of the instant invention will become readily apparent tothose skilled in the art from the following detailed description of apreferred embodiment thereof taken in conjunction with the followingdrawings:

[0013]FIG. 1 is a sectional view of a multi-state magnetoresistiverandom access memory device in accordance with the present invention;

[0014]FIG. 2 is plan view of the multi-state magnetoresistive randomaccess memory device illustrated in FIG. 1 in accordance with thepresent invention;

[0015]FIG. 3 is a graph illustrating the resistance values of amulti-state magnetoresistive random access memory device in the variousstates; and

[0016]FIG. 4 is a graph illustrating the various current pulses used towrite to a multi-state magnetoresistive random access memory device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] Turn now to FIG. 1, which illustrates a simplified sectional viewof a multi-state MRAM cell 5 in accordance with the present invention.Multi-state MRAM cell 5 includes a multi-state MRAM device 7 sandwichedtherebetween a base electrode 14 and a conductive line 36 whereinmulti-state MRAM device 7 has a resistance, R. Further, a conductiveline 12 is positioned proximate to base electrode 14 and an isolationtransistor 10 is electrically connected to base electrode 14 and anelectrical ground 13 as illustrated.

[0018] Multi-state MRAM device 7 includes a pinned syntheticanti-ferromagnetic region 19 positioned adjacent to base electrode 14.Pinned synthetic anti-ferromagnetic region 19 includes ananti-ferromagnetic pinning layer 16 positioned on base electrode 14, apinned ferromagnetic layer 17 positioned on layer 16, ananti-ferromagnetic coupling layer 18 positioned on layer 17, and a fixedferromagnetic layer 22 positioned on layer 18. Further, fixedferromagnetic layer 22 has a fixed magnetic moment vector 20 oriented ina fixed preferred direction (see FIG. 3) at a first nonzero anglerelative to conductive line 36. It will be understood that region 19 canbe substituted by many configurations, including using a single fixedlayer, and the use of four layers in this embodiment is for illustrativepurposes only.

[0019] A non-ferromagnetic spacer layer 24 with a thickness ispositioned on pinned synthetic anti-ferromagnetic region 19. It will beunderstood that non-ferromagnetic spacer layer 24 can include multiplelayers, but is shown as one layer for illustrative purposes. Also, itwill be understood that non-ferromagnetic spacer layer 24 can include adielectric material, such as aluminum oxide (AlO), wherein multi-stateMRAM device 7 behaves as a tunneling junction device. Layer 24 istypically thin enough to allow a spin polarized tunneling current toflow between fixed ferromagnetic layer 22 and ferromagnetic layer 28. Inother embodiments, non-ferromagnetic spacer layer 24 can include aconductive material, such as copper (Cu), wherein multi-state MRAMdevice 7 behaves as a giant magnetoresistive device. In general,however, non-ferromagnetic spacer layer 24 can include any suitablenon-ferromagnetic material which results in a device with a substantialmagnetoresistive ratio.

[0020] A free ferromagnetic region 26 is positioned on non-ferromagneticspacer layer 24 and adjacent to conductive line 36. Ferromagnetic layer28 has a free magnetic moment vector 30 that is free to rotate in thepresence of an applied magnetic field. In the preferred embodiment, freeferromagnetic region 26 is designed to provide free magnetic momentvector 30 with more than two stable positions, as will be discussedpresently. In the preferred embodiment, free magnetoresistive region 26includes a tri-layer structure that includes an anti-ferromagneticcoupling spacer layer 32 sandwiched therebetween a ferromagnetic layer28 and a ferromagnetic layer 34.

[0021] It will be understood that free magnetoresistive region 26 caninclude more than three layers and that the use of three layers in thisembodiment is for illustrative purposes only. For example, a five-layerstack of a ferromagnetic layer/anti-ferromagnetic coupling spacerlayer/ferromagnetic layer/anti-ferromagnetic coupling spacerlayer/ferromagnetic layer could also be used. Further, it will beunderstood that region 26 could include less than three layers such as asingle free ferromagnetic layer.

[0022] In the preferred embodiment, pinned ferromagnetic layer 22 andferromagnetic layer 28 have a split band structure with respect toelectron spin that causes polarization of the conduction band electrons.In general, any material with a split band structure resulting in spinpolarization of the conduction band electrons can be included in layers22 and 28. The spin polarization of these materials results in a devicestructure that depends on the relative orientation of magnetic momentvectors 20 and 30. Further, in some embodiments, at least one of fixedferromagnetic layer 22 and free ferromagnetic layer 28 can includehalf-metallic materials. It is well known to those skilled in the artthat half-metallic materials ideally have 100% spin polarization, but inpractice typically have at least 80% spin polarization. The use ofhalf-metallic materials greatly increases a signal-to-noise ratio ofmulti-state MRAM cell 5.

[0023] Turn now to FIG. 2 which illustrates a simplified plan view ofmulti-state MRAM cell 5. To simplify the description of the operation ofmulti-state MRAM device 7, all directions will be referenced to an x-and y-coordinate system 40 as shown. In coordinate system 40, an angle θis defined to be 0° along the positive x-axis, 90° along the positivey-axis, 180° along the negative x-axis, and 270° along the negativey-axis.

[0024] Further, in the preferred embodiment, a bit current, I_(B), isdefined as being positive if flowing in the positive x-direction and aswitch current, I_(S), is defined as being positive if flowing in thepositive y-direction. The purpose of conductive line 12 and conductiveline 36 is to create a magnetic field that acts upon multi-state MRAMdevice 7. Positive bit current, I_(B), will induce a circumferential bitmagnetic field, H_(B), and positive switch current, I_(S), will induce acircumferential switch magnetic field, H_(S). Since conductive line 36is above multi-state MRAM device 7, in the plane of the element, H_(B)will be applied to multi-state MRAM device 7 in the positive y-directionfor positive bit current, I_(B). Similarly, since conductive line 12 ispositioned below multi-state MRAM device 7, in the plane of the element,H_(S) will be applied to multi-state MRAM device 7 in the positivex-direction for positive switch current, I_(S).

[0025] It will be understood that the definitions for positive andnegative current flow are arbitrary and are defined here forillustrative purposes and convenience. The effect of reversing thecurrent flow is to change the direction of the magnetic field inducedwithin multi-state MRAM device 7. The behavior of a current inducedmagnetic field is well known to those skilled in the art and will not beelaborated upon further here.

[0026] As discussed previously, free ferromagnetic region 26 is designedto provide free magnetic moment vector 30 with more than two stablepositions. One method to create more than two stable positions is tomanipulate the shape of free ferromagnetic region 26. For example, inone embodiment, free ferromagnetic region 26 can have a shape that isdefined by a polar equation given as r(θ)=1+|A·cos(N·θ−α)|, wherein N isapproximately half the number of stable positions and is a whole numbergreater than one, θ is an angle relative to conductive lines 12 and 36,r is a distance in polar coordinates that is a function of the angle θin degrees, α is an angle in degrees that determines the angle of thelobes with respect to conductive line 12, and A is a constant. The angleθ has continuous values in the range between 0° and 360° and, in thisembodiment, the constant A has a value between 0.1 and 2.0.

[0027] In the preferred embodiment, the equation for polar function r(θ)is chosen so that at least one lobe is oriented parallel with conductiveline 12 (i.e. θ=90°). It will be understood that polar function r(θ)traces out the outer edge of free ferromagnetic region 26 to illustratethe basic shape of the region and not the dimensions. Also, it will beunderstood that other equations are possible to describe the basic shapeand that other shapes are possible to induce a shape anisotropy thatcreates more than two stable states.

[0028] In general, however, other methods of inducing an anisotropycould be used alone or in combination with shape anisotropy. Forexample, an intrinsic anisotropy of a magnetic material included in freeferromagnetic region 26, generally thought to arise from atomic-levelpair ordering, can be used. Also, the direction of the intrinsicanisotropy can be set by applying a magnetic field during deposition offree ferromagnetic region 26 or during a post deposition anneal. Amagnetocrystalline anisotropy of the magnetic material included in freeferromagnetic region 26 could also be used by growing a magneticmaterial with a preferred crystalline orientation. Further, ananisotropy induced by certain anisotropic film growth methods could alsobe used to induce an anisotropy wherein the induced anisotropy isthought to originate from a shape asymmetry of the growing clusters orcrystallites.

[0029] However, it will be understood that in the preferred embodiment,the anisotropy of free ferromagnetic region 26 is created by a shapeanisotropy. A shape anisotropy is used in the preferred embodiment forillustrative purposes only and it will be understood that other methodsare available to create an anisotropy, and, consequently, more than twostable states in free ferromagnetic region 26.

[0030] In the preferred embodiment, it is assumed that multi-state MRAMcell 5 illustrated in FIG. 2 has four stable states wherein N is equalto four, A is equal to 0.5, and α is equal to 180°, and that the fourstable states are created by the shape anisotropy of free ferromagneticregion 26. Further, it is assumed that the shape of freemagnetoresistive region 26 induces an easy axis 44 and an easy axis 42which are oriented at a nonzero angle relative to one another, whereinthe nonzero angle is 90° in the preferred embodiment. Further, easy axis44 and easy axis 42 are oriented at a nonzero angle relative to pinnedmagnetic moment vector 20 (not shown), conductive line 36, andconductive line 12.

[0031] Also in the preferred embodiment, it is assumed that the shape offree magnetoresistive region 26 induces a hard axis 46 and a hard axis48 wherein hard axis 46 is oriented parallel to conductive line 36 andhard axis 48 is oriented parallel to conductive line 12. It will beunderstood that the magnetization directions of the stable states may becomplex, with the magnetization bending or curling to minimize itsenergy, but it is assumed for illustrative purposes that themagnetization directions in this embodiment are oriented 90° apart alongeasy axes 42 and 44, as discussed above.

[0032] Further, it will be understood that the magnetization is notgenerally uniform in the same direction over the area of the bit, but isassumed to be uniform in this embodiment for simplicity. Thus, forsimplicity, the easy axis is defined as being an axis which is orientedwith a center of MRAM cell 5 when the magnetic moment vector is in astable rest state.

[0033] In this embodiment, easy axis 44 creates a stable position 50 anda stable position 56 and easy axis 42 creates a stable position 52 and astable position 54. Hence, stable position 54 is oriented 90° relativeto stable position 56, stable position 50 is oriented 180° relative tostable position 56, and stable position 52 is oriented 270° relative tostable position 56. Thus, in this embodiment, four stable positions havebeen created in free magnetoresistive region 26 for free magnetic momentvector 30 to align with. It will be understood that the angles betweenadjacent stable positions are chosen to be 90° for illustrative purposesand other angles could be chosen.

[0034] The relationships between free magnetic moment vector 30 and theresistance are illustrated in graph 81 in FIG. 3 where free magneticmoment vector 30 is shown oriented in stable positions 50, 52, 54, and56 along a R-axis. The R-axis is defined as a resistance axis directedanti-parallel with the direction of pinned magnetic moment vector 20.

[0035] Resistance, R, of multi-state MRAM cell 5 depends on the positionof free magnetic moment vector 30 relative to pinned magnetic momentvector 20. It is well known by those skilled in the art that theresistance of a magnetoresistive device, such as a magnetic tunneljunction or a spin valve device, varies between a minimum value,R_(min), and a maximum value, R_(max), by approximately as the cosine ofan angle, φ, between magnetic moment vectors 20 and 30 according to therelationship given as$R \approx {{\frac{1}{2}\left( {R_{\min} + R_{\max}} \right)} - {\frac{1}{2}\left( {R_{\max} - R_{\min}} \right){{\cos (\varphi)}.}}}$

[0036] R is at its maximum value, R_(max), when vectors 20 and 30 areanti-parallel (i.e. φ=180°) so that cos(φ))=−1. R is at its minimumvalue, R_(min), when vectors 20 and 30 are parallel (i.e. φ=0°) so thatcos(φ))=1. It is further understood by those skilled in the art that ifone of layers 22 or 28 has an opposite polarization of the conductionelectrons then the principles of the device are unchanged, but the signof the cosine dependence is opposite. This mathematical cosinerelationship is shown graphically as the projection of the magneticmoment vector 20 on the R-axis in graph 81.

[0037] In the preferred embodiment, R has a value R₁₁ when free magneticmoment vector 30 is in stable position 56, a value R₀₁ when freemagnetic moment vector 30 is held in stable position 54, a value R₀₀when free magnetic moment vector is held in stable position 50, and avalue R₁₀ when free magnetic moment vector is held in stable position52. Further, it will be understood that in this illustrationR₀₀<R₀₁<R₁₀<R₁₁.

[0038] For example, if free magnetic moment vector 30 is oriented instable position 50, then multi-state MRAM device 7 has a resistancevalue of R₀₀, which is the projection of free magnetic moment vector 30in stable position 50 onto the R-axis. Similarly, the projections ofmagnetic moment vector 30 in stable positions 52, 54, and 56 havecorresponding resistance values of R₁₀, R₀₁, and R₁₁, respectively.Thus, the state of multi-state MRAM device 7 can be read by measuringits resistance.

[0039] The method of writing to multi-state MRAM cell 5 involvessupplying currents in conductive line 12 and conductive line 36 suchthat free magnetic moment vector 30 can be oriented in one of fourstable positions in the preferred embodiment. To fully elucidate thewriting method, specific examples describing the time evolution ofcircumferential bit magnetic field, H_(B), and circumferential switchmagnetic field, H_(S), are now given.

[0040] Turn now to FIG. 4 which illustrates a magnetic pulse sequence100. Illustrated are the magnetic pulse sequences used to writemulti-state MRAM cell 5 to various states. The writing method involvesapplying current pulses in conductive line 12 and conductive line 36 torotate free magnetic moment vector 30 in a direction parallel to one ofthe four stable positions in the preferred embodiment. The currentpulses are then turned off so that free magnetic moment vector 30 isaligned in one of the four stable positions.

[0041] It will be understood that for a memory array one can arrange aplurality of MRAM cells on a grid with lines 12 and 36 electricallyconnected to an array of cells arranged in rows and columns to form across point array. With multi-state MRAM cells, as with conventional twostate MRAM cells, the bits ideally must not switch when exposed to afield generated by a single conductive line. It is desired to have onlythe bit which is at the cross-point of two active conductive lines beswitched.

[0042] In the preferred embodiment, MRAM cell 5 requires a much highermagnetic field to switch 180° compared to 90°. The currents used forwriting are designed to generate magnetic fields that are above thethreshold for 90° switching but below the threshold required for 180°switching. In this way a sequence of current pulses can be applied tothe lines that will switch only the MRAM cell at the cross-point andwill move its magnetic moment vector in a sequence of 0° and/or 90°switches until it reaches the desired final state regardless of theinitial state.

[0043] In FIG. 4, for example, the longer pulses of H_(B) determine ifthe final state will be in the negative or positive y-direction whilethe bipolar pulse of H_(S) determines if the final state will be in thepositive or negative x-direction. Due to the symmetry of MRAM cell 5, anequivalent writing scheme can be constructed by reversing the roles ofH_(S) and H_(B) so that H_(S) pulses are long and H_(B) pulses are shortand bipolar. If δ is the duration of a short pulse, δ=t_(i+1)−t₁, thenthe total duration of the write cycle shown in FIG. 4 is 4·δ. Since theactive part of the cycle for any given bit is only the part of the cyclewith the bipolar pulses on H_(S), the same result could be obtained withslightly different write circuitry that only executes the active part ofthe cycle needed to write the desired state. Thus the total cycle timecan be reduced to 2·δ.

[0044] It is understood by those skilled in the art that the currentpulses in a circuit may have variations in shape and duration, such asfinite rise time, overshoot, and finite separation between pulses andtherefore may not appear exactly as illustrated in FIG. 4.

[0045] One principle of operation in writing to MRAM cell 5 is having asingle long pulse on one conductive line coincident with a bipolar pulseon another conductive line to move the magnetic moment of the MRAM bitto the desired state. In particular, a finite separation between thecurrent pulses may be desirable so that the magnetic state of the freelayer moves to an equilibrium state before the beginning of the nextcurrent pulse.

[0046] For example, by using a magnetic pulse sequence 102 for H_(S) anda magnetic pulse sequence 112 for H_(B), multi-state MRAM cell iswritten into a ‘11’ state. In particular, at a time t₀, H_(B) is pulsedto a negative value while H_(S) is zero. Since only one write line ison, the magnetic moment of the MRAM bit will not change states.

[0047] At a time t₂, H_(B) is pulsed to a positive value while H_(S) ispulsed to a negative value. If the initial state was in direction 56 or50, then this will cause a 90 rotation of the magnetic moment direction54. If the initial state is 54 or 52, then there will be no rotation. Ata time t₃ when the negative H_(S) pulse is complete, the bit can only beoriented with its magnetic moment substantially along axis 42 in FIG. 2.At time t₃, H_(B) is kept at its positive value while H_(S) is pulsed toa positive value. This has the effect of orienting free magnetic momentvector 30 toward direction 56. At a time t₄, H_(B) and H_(S) are bothmade zero so a magnetic field force is not acting upon free magneticmoment vector 30 and the moment settles into equilibrium position 56. Itwill be understood that in this illustration t₀<t₁<t₂<t₃<t₄.

[0048] Consequently, free magnetic moment vector 30 will become orientedin the nearest stable position to minimize the anisotropy energy, whichin this case is stable position 56. As discussed previously andillustrated graphically in FIG. 3, stable position 56 is defined as the‘11’ state. Hence, multi-state MRAM cell 5 has been programmed to storea ‘11’ by using magnetic pulse sequences 102 and 112. Similarly,multi-state MRAM cell 5 can be programmed to store a ‘10’ by usingmagnetic pulse sequences 104 and 112, to store a ‘01’ by using magneticpulse sequences 106 and 112, and to store a ‘00’ by using magnetic pulsesequences 108 and 112. It will be understood that the states used inthis illustration are arbitrary and could be otherwise defined.

[0049] Thus, multi-state MRAM cell 5 can be programmed to store multiplestates without decreasing the dimensions of multi-state MRAM cell 5.Consequently, in the preferred embodiment, the memory storage density isincreased by a factor of two. Also, a writing method has beendemonstrated in the preferred embodiment so that one of four possiblestates can be stored in the MRAM device regardless of the initial storedstate.

[0050] Various changes and modifications to the embodiments hereinchosen for purposes of illustration will readily occur to those skilledin the art. To the extent that such modifications and variations do notdepart from the spirit of the invention, they are intended to beincluded within the scope thereof which is assessed only by a fairinterpretation of the following claims.

[0051] Having fully described the invention in such clear and conciseterms as to enable those skilled in the art to understand and practicethe same, the invention claimed is:

1. A multi-state magnetoresistive random access memory device having aresistance, the device comprising: a substrate; a first conductive linepositioned on the substrate; a fixed ferromagnetic region positionedproximate to the first conductive line, the fixed ferromagnetic regionhaving a fixed magnetic moment vector fixed in a preferred directionboth with and without an applied magnetic field; a non-ferromagneticspacer layer positioned on the pinned ferromagnetic region; a freeferromagnetic region positioned on the non-ferromagnetic spacer layer,the free ferromagnetic region having a free magnetic moment vector thatis free to rotate in the presence of an applied magnetic field and wherethe free ferromagnetic region has an anisotropy that creates more thantwo stable positions for the free magnetic moment vector wherein themore than two stable positions are oriented relative to the preferreddirection of the fixed magnetic moment vector such that each positionproduces a unique resistance; and a second conductive line positioned ata non-zero angle to the first conductive line.
 2. An apparatus asclaimed in claim 1 wherein the anisotropy of the free ferromagneticregion is created by the shape of the free ferromagnetic region.
 3. Anapparatus as claimed in claim 2 wherein the shape of the freeferromagnetic region is defined approximately by a polar equation givenas r(θ)=1+|A·cos(N·θ−α)|, wherein N is a whole number, θ is an angle indegrees relative to the first and second conductive lines, r is adistance in polar coordinates that is a function of the angle θ, α is anangle in degrees that determines the angle of the lobes relative to thefirst conductive line, and A is a constant.
 4. An apparatus as claimedin claim 3 wherein the constant A has a value between 0.1 and 2.0.
 5. Anapparatus as claimed in claim 2 wherein the shape of the freeferromagnetic layer has at least one lobe oriented parallel to the firstconductive line.
 6. An apparatus as claimed in claim 3 wherein the angleθ has continuous values in the range between 0° and 360°.
 7. Anapparatus as claimed in claim 1 wherein at least one of the pinnedferromagnetic region and the free ferromagnetic region includes one ofiron oxide (Fe₃O₄), chromium oxide (CrO₂), oxides, and semiconductors,wherein the oxides and semiconductors are doped with one or more ofiron, cobalt, nickel, magnesium, and chromium.
 8. An apparatus asclaimed in claim 1 wherein the anisotropy of the free ferromagneticregion is created by an intrinsic anisotropy of a material included inthe free ferromagnetic region.
 9. An apparatus as claimed in claim 8wherein the intrinsic anisotropy is created by applying a magnetic fieldto the free ferromagnetic region.
 10. An apparatus as claimed in claim 1wherein the anisotropy of the free ferromagnetic region is created bygrowing the free ferromagnetic region with a preferred crystallineorientation.
 11. An apparatus as claimed in claim 1 wherein theanisotropy of the free ferromagnetic region is created by growing thefree ferromagnetic region with clusters or crystallites that have ashape anisotropy.
 12. An apparatus as claimed in claim 1 wherein themore than two stable positions are oriented at a nonzero angle relativeto the first and second conductive lines.
 13. An apparatus as claimed inclaim 1 wherein the magnetic moment of the fixed ferromagnetic region isat a non-zero angle relative to the first and second conductive lines.14. An apparatus as claimed in claim 1 wherein the stable positions areinduced by a shape anisotropy of the free ferromagnetic region.
 15. Anapparatus as claimed in claim 1 wherein the anisotropy of the freeferromagnetic region is created by the free ferromagnetic region havingclusters or crystallites that have a shape anisotropy.
 16. A multi-statemagnetoresistive random access memory device comprising: a substrate; afirst conductive line positioned proximate to the base electrode; a freeferromagnetic region positioned proximate to the first conductive line,the free ferromagnetic region having a free magnetic moment vector thatis free to rotate in the presence of an applied magnetic field and wherethe free ferromagnetic region has an anisotropy that creates more thantwo stable positions for the free magnetic moment vector wherein themore than two stable positions are oriented relative to the preferreddirection of the fixed magnetic moment vector such that each positionproduces a unique resistance; a non-ferromagnetic spacer layerpositioned on the free ferromagnetic region; a fixed ferromagneticregion positioned on the non-ferromagnetic spacer layer, the fixedferromagnetic region having a fixed magnetic moment vector fixed in apreferred direction both with and without an applied magnetic field; anda second conductive line positioned at a non-zero angle to the firstconductive line.
 17. An apparatus as claimed in claim 16 wherein theshape of the free ferromagnetic layer has at least one lobe orientedparallel to the first conductive line.
 18. An apparatus as claimed inclaim 14 wherein the shape of the free ferromagnetic region is definedapproximately by a polar equation given as r(θ)=1+|A·cos(N·θ−α)|,wherein N is a whole number, θ is an angle in degrees relative to thefirst and second conductive lines, r is a distance in polar coordinatesthat is a function of the angle θ, α is an angle in degrees thatdetermines the angle of the lobes relative to the first conductive line,and A is a constant.
 19. An apparatus as claimed in claim 18 wherein theconstant A has a value between 0.1 and 2.0.
 20. An apparatus as claimedin claim 18 wherein the angle θ has continuous values in the rangebetween 0° and 360°.
 21. An apparatus as claimed in claim 18 wherein thestable positions are induced by a shape anisotropy of the freeferromagnetic region.
 22. An apparatus as claimed in claim 18 whereinthe number of stable positions is equal to four.
 23. An apparatus asclaimed in claim 16 wherein the anisotropy of the free ferromagneticregion is created by an intrinsic anisotropy of a material included inthe free ferromagnetic region.
 24. An apparatus as claimed in claim 23wherein the intrinsic anisotropy is created by applying a magnetic fieldto the free ferromagnetic region.
 25. An apparatus as claimed in claim16 wherein the anisotropy of the free ferromagnetic region is created bygrowing the free ferromagnetic region with a preferred crystallineorientation.
 26. An apparatus as claimed in claim 16 wherein theanisotropy of the free ferromagnetic region is created by growing thefree ferromagnetic region with clusters or crystallites that have ashape anisotropy.
 27. An apparatus as claimed in claim 16 wherein atleast one of the pinned ferromagnetic region and the free ferromagneticregion includes one of iron oxide (Fe₃O₄), chromium oxide (CrO₂),oxides, and semiconductors, wherein the oxides and semiconductors aredoped with one or more of iron, cobalt, nickel, magnesium, and chromium.28. An apparatus as claimed in claim 16 wherein the non-ferromagneticspacer layer includes one of a dielectric material, such as aluminumoxide, and a conductive material, such as copper.
 29. An apparatus asclaimed in claim 16 wherein the more than two stable positions areoriented at a nonzero angle relative to the first and second conductivelines.
 30. A method of fabricating a multi-state magnetoresistive randomaccess memory device comprising the steps of: providing a substratedefining a surface; supporting a base electrode on the surface of thesubstrate; forming a material layer between a fixed magnetoresistiveregion having a fixed magnetic moment vector fixed in a preferreddirection both with and without an applied magnetic field and a freeferromagnetic region with a shape wherein the free ferromagnetic regionis designed to provide a free magnetic moment vector with more than twostable positions, wherein the more than two stable positions areoriented at a nonzero-angle relative to the preferred direction of thefixed magnetic moment vector; forming a bit conductive line, wherein oneof the material layer and the free ferromagnetic region is positioned onthe base electrode and the bit conductive line is positioned on theother of the material layer and the free ferromagnetic region.
 31. Amethod as claimed in claim 30 further including the step of positioninga digit conductive line proximate to the base electrode.
 32. A method asclaimed in claim 30 further including the step of connecting anisolation transistor to the base electrode and an electrical ground. 33.A method as claimed in claim 30 wherein the material layer includes oneof aluminum oxide and another suitable dielectric material.
 34. A methodas claimed in claim 30 wherein the material layer includes one of copperand another suitable conductive material.
 35. A method as claimed inclaim 30 wherein the more than two stable positions are created by ananisotropy of the free ferromagnetic region.
 36. A method as claimed inclaim 35 wherein the anisotropy is created by a shape anisotropy of thefree ferromagnetic region.
 37. A method as claimed in claim 35 whereinthe anisotropy is created by an intrinsic anisotropy of a materialincluded in the free ferromagnetic region.
 38. A method as claimed inclaim 37 wherein the intrinsic anisotropy is created by applying amagnetic field to the free ferromagnetic region.
 39. A method as claimedin claim 35 wherein the anisotropy is created by growing the freeferromagnetic region with a preferred crystalline orientation.
 40. Amethod as claimed in claim 35 wherein anisotropy is created by growingthe free ferromagnetic region with clusters or crystallites that have ashape anisotropy.
 41. A method as claimed in claim 30 wherein at leastone of the pinned ferromagnetic region and the free ferromagnetic regionincludes one of iron oxide (Fe₃O₄), chromium oxide (CrO₂), oxides, andsemiconductors, wherein the oxides and semiconductors are doped with oneor more of iron, cobalt, nickel, magnesium, and chromium.
 42. A methodas claimed in claim 30 wherein the more than two stable positions areoriented at a nonzero angle relative to the first and second conductivelines.
 43. A method of storing multiple states in a multi-statemagnetoresistive random access memory device comprising the steps of:providing a multi-state magnetoresistive random access memory deviceadjacent to a first conductor and a second conductor wherein themulti-state magnetoresistive random access memory device includes afirst ferromagnetic region and a second ferromagnetic region separatedby a non-ferromagnetic spacer layer, at least one of the first andsecond ferromagnetic regions include a synthetic anti-ferromagneticregion and has a free magnetic moment vector oriented in a preferreddirection at a time t₀, wherein at least one of the first and secondferromagnetic regions has an anisotropy that provides the free magneticmoment vector with more than two stable positions, and wherein at leastone of the first and second ferromagnetic regions has a fixed magneticmoment vector wherein the fixed magnetic moment vector is oriented in apreferred direction both with and without an applied magnetic fieldwherein the more than two stable positions are oriented at a nonzeroangle relative to the preferred direction of the fixed magnetic momentvector; applying a first and a second current pulse to orient the freemagnetic moment vector in one of the more than two stable positions; andturning off the first and second current pulses so that the freemagnetic moment vector is aligned with one of N stable positions in theabsence of an applied magnetic field.
 44. A method as claimed in claim43 further including the step of orientating the first and secondconductors at a 90° angle relative to each other.
 45. A method asclaimed in claim 43 further including the step of setting the preferreddirection at the time t₀ to be at a non-zero angle to the first andsecond conductors.
 46. A method as claimed in claim 43 wherein the stepof causing a first and second current flow in the first and secondconductors, respectively, includes using a combined current magnitudethat is large enough to cause the magnetoresistive memory element toswitch.
 47. A method as claimed in claim 43 wherein the more than twostable positions are created by an anisotropy of the free ferromagneticregion.
 48. A method as claimed in claim 47 wherein the anisotropy iscreated by a shape anisotropy of at least one of the first and secondmagnetoresistive regions.
 49. A method as claimed in claim 48 whereinthe shape has at least one lobe oriented parallel with one of the firstconductive line and the second conductive line.
 50. A method as claimedin claim 43 wherein the anisotropy is created by an intrinsic anisotropyof a material included in at least one of the first and secondmagnetoresistive regions.
 51. A method as claimed in claim 50 whereinthe intrinsic anisotropy is created by applying a magnetic field to atleast one of the first and second magnetoresistive regions.
 52. A methodas claimed in claim 43 wherein the anisotropy is created by growing atleast one of the first and second magnetoresistive regions with apreferred crystalline orientation.
 53. A method as claimed in claim 43wherein anisotropy is created by growing at least one of the first andsecond magnetoresistive regions with clusters or crystallites that havea shape anisotropy.
 54. A method as claimed in claim 43 wherein the morethan two stable positions are oriented at a nonzero angle relative tothe first and second conductive lines.
 55. A method as claimed in claim43 wherein the step of causing a first and second current flow in thefirst and second conductors, respectively, includes using a combinedcurrent magnitude that is large enough to allow the magnetoresistivememory element to switch the free magnetic moment vector to a firstposition but not a second position.
 56. A multi-state magnetoresistiverandom access memory device having a resistance, the device comprising:a first ferromagnetic region; a second ferromagnetic region positionedadjacent to the first ferromagnetic region to form a magnetoresistivememory device wherein one of the first and second ferromagnetic regionsincludes a shape having more than two magnetic moment vector stablepositions, the stable positions being induced by an anisotropy of amaterial included within one of the first and second ferromagneticregions.